How to Calculate Flow and Head for a Submersible Sewage Pump

Choosing a submersible sewage pump becomes much easier once two numbers are known: how much wastewater needs to be pumped and how much total head the pump must overcome. These are usually shown as flow in m³/h and head in metres. The difficulty is that neither value should be guessed from the motor power, discharge pipe size or the maximum figures printed in a catalogue.

For a sewage pumping system, the required flow depends on how quickly wastewater enters the pit or pumping station and how the system is expected to operate. The required head depends on the difference in liquid level between the suction and discharge sides, together with the resistance created by the discharge pipeline, valves and fittings. Once these values are calculated, they form the pump's actual duty point. If you are still deciding what type of pump is suitable for the wastewater itself, read our submersible sewage pump buying guide first, because solids, fibres, corrosion and impeller design also affect the final selection.

1. Start by Finding the Wastewater Flow You Actually Need to Pump

The required pump flow should come from the amount of wastewater entering the system, not from the maximum flow of a pump model that happens to fit the pipe. In a residential or commercial sewage system, the inflow may change throughout the day. An industrial plant may discharge wastewater according to production shifts, washing cycles or batch processes. Stormwater and drainage systems can have even larger differences between normal and peak conditions. This is why the design should consider the maximum realistic inflow that the pumping system is expected to handle, while also taking into account the usable volume of the wet well and the way the pumps are controlled.

For example, if a wastewater pit normally receives 25 m³/h but reaches 60 m³/h during the highest expected operating period, choosing the pump only around the 25 m³/h average can allow the liquid level to continue rising when inflow reaches its peak. At the same time, simply installing a 150 m³/h pump is not automatically better. An unnecessarily large pump can empty a small pit very quickly and then restart as soon as the level rises again, creating frequent start-stop cycles. In a well-designed system, pump capacity, wet-well storage, start and stop levels and expected inflow are considered together. For larger municipal or industrial installations, several pumps may also operate alternately or in parallel rather than relying on a single oversized unit.

2. Understand What Sewage Pump Head Really Means

Once the flow requirement is known, the next step is to calculate the head. For most sewage transfer systems, the total head is made up mainly of static head and friction head. Static head is created by the elevation and pressure difference between the liquid at the source and the final discharge point. Friction head is the energy lost while the wastewater moves through pipes, elbows, valves, check valves and other components. Hydraulic Institute describes the system head at a given flow as the combination of static conditions and frictional losses, and this head changes with flow because friction increases as velocity increases.

A useful simplified relationship for many sewage pumping systems is:

Total Dynamic Head = Static Head + Pipe and Fitting Friction Loss + Required Discharge Pressure, if any

This means that the depth of the sewage pit is not automatically the pump head. If a pump sits 6 metres below the liquid surface but the wastewater only needs to rise 8 metres from the operating liquid level to the discharge point, the static lift is based on that difference in liquid elevation rather than simply adding the pump installation depth. The same principle applies whether the pump is used in a small collection pit or as part of a larger submersible sewage pumping system.

3. Calculate the Static Head from the Actual Liquid Levels

In a typical sewage lifting system, static head is the vertical difference between the liquid level on the suction side and the liquid level or pressure condition at the discharge destination. Imagine a wet well where the normal pumping level is 2 metres below ground and the discharge pipe finally enters an open receiving chamber 10 metres above ground level. The approximate static head is therefore 12 metres. The fact that the pump itself may be installed another 2 or 3 metres below the operating water level does not mean those metres should automatically be added again.

The calculation should also consider how the liquid levels change during operation. As the wet-well level falls between the pump's start and stop points, the static head increases slightly. If the receiving tank or downstream water level also changes, the system may have more than one operating condition. For an important project, it is better to calculate both the lower-head and higher-head conditions rather than selecting the pump from one convenient figure. If the discharge is connected to a pressurized pipeline instead of an open chamber, the pressure already present in that pipeline must also be converted to head and included in the calculation.

4. Add the Pressure Lost Through the Discharge Pipeline

Pipe friction is often the part of sewage pump selection that is most easily underestimated. Every metre of pipe creates some resistance to flow, and additional losses occur through elbows, tees, reducers, check valves, isolation valves and other fittings. Friction loss becomes larger as flow velocity rises, so the same pipeline does not have one fixed friction value for every operating condition. Hydraulic Institute specifically identifies pipe diameter, fittings and valves as major contributors to friction head, with friction changing as the velocity through the system changes.

This matters when deciding the discharge pipe diameter. A smaller pipe may look attractive because the material cost is lower, but forcing the same wastewater flow through a smaller internal diameter increases velocity and can increase friction considerably. The pump then has to produce more head for the same useful flow, which may increase power consumption and change the actual operating point. On the other hand, sewage piping cannot be sized only to minimize friction; the design must also maintain suitable transport conditions for the solids contained in the wastewater. For serious municipal and industrial projects, the pipe diameter and pump should therefore be evaluated as one hydraulic system rather than as separate purchases.

Friction loss can be calculated using established hydraulic equations such as Darcy-Weisbach or with engineering software and verified pipe-loss data. To obtain a meaningful result, the calculation needs the design flow, actual internal pipe diameter, pipe length, pipe material and the quantity and type of major fittings. A statement such as “the pipe is about 100 metres long” is not enough for an accurate pump selection if the pipe diameter and operating flow are unknown.

5. Put Flow and Head Together to Find the Pump Duty Point

Consider a simple wastewater transfer example. The required pumping capacity is 80 m³/h. At the design operating condition, the difference between the sewage level in the wet well and the receiving point is 14 metres. After calculating the discharge pipe, check valve, elbows and other fittings at 80 m³/h, the total friction loss is estimated at 6 metres. The wastewater is discharged into an open receiving system, so there is no additional pressure requirement.

The required total dynamic head is therefore:

TDH = 14 m static head + 6 m friction loss

TDH = 20 m

The design duty point is:

80 m³/h at 20 m head

This does not mean the buyer should simply search for a pump with a maximum flow above 80 m³/h and maximum head above 20 metres. A centrifugal pump produces different head at different flow rates. The correct submersible sewage pump for wastewater transfer should have a performance curve that actually reaches approximately 80 m³/h while producing approximately 20 metres of head. The real operating point is determined by the relationship between the pump curve and the system curve; Hydraulic Institute defines it as the point where the pump and system characteristics intersect.

6. Do Not Select a Sewage Pump from Maximum Flow or Maximum Head

Pump catalogues commonly show a flow range and a head range, but the highest flow and highest head normally do not occur at the same operating point. A pump may be capable of producing a high head near the low-flow end of its curve and a much higher flow when the system head is lower. Taking the maximum flow from one end of the curve and the maximum head from the other and treating them as one duty point creates a pump specification that the pump may never actually achieve.

This is why the performance curve should be one of the first documents checked when comparing sewage pumps. The selected point should fall within the manufacturer's recommended operating range rather than being located at an extreme end of the curve. Operating far away from the intended region can reduce efficiency and reliability, while oversizing a centrifugal pump may create excessive flow, unnecessary throttling and higher mechanical loading. Hydraulic Institute notes that oversized pumps operating away from their appropriate region can contribute to higher loads and premature wear of components such as bearings and seals.

The hydraulic curve is only one part of sewage pump selection. The wastewater must still be able to pass through the pump without unacceptable clogging or wear. A pump that perfectly matches 80 m³/h at 20 metres but has the wrong impeller for long fibres, wipes or abrasive solids is still the wrong pump. For this reason, the duty point should be combined with an assessment of the wastewater characteristics before the final model is chosen.

7. Flow and Head Can Change When More Than One Pump Operates

Many sewage pumping stations use two or more pumps so that capacity can be adjusted as inflow changes or so that another unit is available when one pump is out of service. It is important to understand that two identical pumps operating in parallel do not necessarily produce exactly twice the flow of one pump. When the combined flow increases, velocity through the common discharge pipeline also increases, which raises friction loss and changes the system operating point. Hydraulic Institute specifically notes that increasing parallel-pump flow can increase friction head and cause the resulting flow from each pump to differ from a simple two-times calculation.

For a small duty/standby system where only one pump normally operates, the single-pump duty point may be relatively straightforward to determine. For a station where two or three pumps are expected to run simultaneously, the supplier should check the individual pump curves, combined pump curve and system curve together. The minimum and maximum wet-well levels should also be included because they change the static head. This is particularly important for municipal wastewater treatment, industrial effluent transfer and other applications where the pumping station needs to perform reliably over a wide range of incoming flows. Our wastewater pumping solutions cover these types of applications, but the hydraulic selection still needs to be based on the actual project data rather than a standard flow-and-head combination.

8. Remember That Wastewater Properties Can Affect the Final Selection

The calculation above works well as a starting point for ordinary wastewater with hydraulic properties reasonably close to water, but not every liquid behaves the same way. Wastewater containing a high concentration of sludge, grease or other viscous material can behave differently from clean water in both the pipeline and pump. Hydraulic Institute notes that liquid viscosity can change rotodynamic pump performance and may require corrections to water-based pump performance data.

Solids also affect the type of pump that should be used even when the calculated flow and head remain unchanged. Domestic sewage containing fibres and wipes may require a non-clog hydraulic design, while wastewater with corrosive chemicals may require a corrosion-resistant stainless steel submersible wastewater pump. Wastewater containing large amounts of heavy grit or dense abrasive sludge may be better suited to a sludge or slurry pumping design rather than a conventional sewage pump. Clog resistance should also not be judged only by a quoted solids-passage diameter; Xylem's published wastewater guidance notes that hydraulic design is an important part of clog resistance and that throughlet size alone can be misleading.

What Information Should You Provide for Sewage Pump Selection?

For a reliable quotation, provide the expected normal and peak flow, the minimum and maximum liquid levels in the wet well, elevation of the final discharge point, discharge pipe diameter and length, pipe material, major valves and fittings and any pressure present at the outlet. The supplier should also know what the wastewater contains, including the approximate solid size, fibres, sand, sludge, temperature and corrosive chemicals. Finally, include the available voltage, frequency, installation method and whether one or several pumps will operate at the same time.

If the exact flow is not yet known, explain where the wastewater comes from and how the system currently operates instead of inventing a number. If friction loss has not been calculated, provide the pipeline information so it can be checked. A technically useful pump quotation should ultimately identify a clear duty point—such as 80 m³/h at 20 m head—and show that point on the proposed pump performance curve.

Conclusion

Calculating a submersible sewage pump starts with two separate questions: how much wastewater must be moved, and how much resistance must the pump overcome while moving it? The first determines the required flow. The second determines the total dynamic head.

For many sewage transfer systems, the basic relationship is:

Total Dynamic Head = Static Head + Friction Loss + Additional Discharge Pressure

Once the required flow and total head are known, they should be considered together as a single operating point and checked against the pump performance curve. The pump should then be matched to the actual wastewater, including solids, fibres, abrasion and corrosion.

Selecting a pump in this order is much more reliable than comparing motor power, outlet diameter, maximum flow or maximum head separately. If you are planning a wastewater pumping project, provide the flow or incoming wastewater volume, liquid levels, discharge elevation, pipeline information and wastewater characteristics so the pump can be selected around the real operating conditions.